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But even planting trees in forest habitats can lead to negative environmental outcomes. From the perspective of both carbon
sequestration and biodiversity, all forests are not equal – naturally established forests contain more species of plants and
animals than plantation forests. They often hold more carbon, too. But policies aimed at promoting tree planting can
unintentionally incentivise deforestation of well established natural habitats.
A recent high-profile example concerns the Mexican government’s Sembrando Vida programme, which provides direct payments
to landowners for planting trees. The problem? Many rural landowners cut down well established older forest to plant seedlings.
This decision, while quite sensible from an economic point of view, has resulted in the loss of tens of thousands of hectares of
mature forest.
This example demonstrates the risks of a narrow focus on trees as carbon absorption machines. Many well meaning
organisations seek to plant the trees which grow the fastest, as this theoretically means a higher rate of CO
₂
“drawdown” from
the atmosphere.
Yet from a climate perspective, what matters is not how quickly a tree can grow, but how much carbon it contains at maturity,
and how long that carbon resides in the ecosystem. As a forest ages, it reaches what ecologists call a “steady state” – this is when
the amount of carbon absorbed by the trees each year is perfectly balanced by the CO
₂
released through the breathing of the
plants themselves and the trillions of decomposer microbes underground.
This phenomenon has led to an erroneous perception that old forests are not useful for climate mitigation because they are no
longer growing rapidly and sequestering additional CO
₂
. The misguided “solution” to the issue is to prioritise tree planting ahead
of the conservation of already established forests. This is analogous to draining a bathtub so that the tap can be turned on full
blast: the flow of water from the tap is greater than it was before – but the total capacity of the bath hasn’t changed. Mature
forests are like bathtubs full of carbon. They are making an important contribution to the large, but finite, quantity of carbon
that can be locked away on land, and there is little to be gained by disturbing them.
What about situations where fast growing forests are cut down every few decades and replanted, with the extracted wood used
for other climate-fighting purposes? While harvested wood can be a very good carbon store if it ends up in long lived products
(like houses or other buildings), surprisingly little timber is used in this way.
Similarly, burning wood as a source of biofuel may have a positive climate impact if this reduces total consumption of fossil fuels.
But forests managed as biofuel plantations provide little in the way of protection for biodiversity and some research questions
the benefits of biofuels for the climate in the first place.
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